Method and apparatus for detection and classification of impairments on an RF modulated network

ABSTRACT

A method is disclosed for maintaining the integrity of a communication system. The method comprises detecting common path distortion (CPD) and periodic impulse/burst (PIB) noise in a received signal in the communication system. After isolating the CPD and PIB noise, ingress noise in the received signal is identified. Isolating the CPD and PIB noise thus prevents improper classification of CPD and PIB noise as ingress noise. Operating parameters of the communication system are then adapted in accordance with the identified ingress noise, the detected CPD and/or the detected PIB noise.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. patent application Ser. No.09/988,065, filed Nov. 16, 2001, and entitled “Method and Apparatus forthe Detection and Classification of Impairments on an RF ModulatedNetwork,” which claims priority to U.S. Patent Provisional ApplicationSer. No. 60/249,111, filed Nov. 16, 2000, both by Daniel H. Howard andboth of which are incorporated by reference in their entirety herein.

This application is related to U.S. patent application Ser. No.09/574,558, entitled “Cable Modem Apparatus and Method,” filed May 19,2000, the content of which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to telecommunications systemsand, more particularly, to shared access RF networks.

2. Background

In conventional shared access communication networks, such as a hybridfiber coaxial (HFC) network, a bidirectional communication path ismaintained between a network headend and each remote point in thenetwork. The communication path simultaneously carries broadband radiofrequency (RF) signals in two directions on the same medium by dividingthe frequency spectrum of the bidirectional communication path.Frequency division multiplexing (FDM) allows two or more simultaneousand continuous channels to be derived from a shared access transmissionmedium. FDM assigns separate portions of the available frequencyspectrum to the “downstream” or “forward path” direction from a headendsignal source to a plurality of remote points, and a second frequencyrange for carrying signals in the “upstream” or “return path” directionfrom each remote point to the headend.

For example, a conventional cable modem system provides apoint-to-multipoint topology for supporting data communication between acable modem termination system (CMTS) at a cable headend and multiplecable modems (CM) at the customer premises. In such systems, informationis broadcast on downstream channels from the CMTS to the cable modems asa continuous transmitted signal in accordance with a time divisionmultiplexing (TDM) technique. In contrast, information is transmittedupstream from each of the cable modems to the CMTS on the upstreamchannels as short burst signals in accordance with a time domainmultiple access (TDMA) technique. The upstream transmission of data fromthe cable modems is managed by the CMTS, which allots to each cablemodem specific slots of time within which to transfer data.

Conventional cable modem systems utilize DOCSIS-compliant equipment andprotocols to carry out the transfer of data packets between multiplecable modems and a CMTS. The term DOCSIS (Data Over Cable SystemInterface Specification) generally refers to a group of specificationspublished by CableLabs that define industry standards for cable headendand cable modem equipment. In part, DOCSIS sets forth requirements andobjectives for various aspects of cable modem systems includingoperations support systems, management, data interfaces, as well asnetwork layer, data link layer, and physical layer transport for dataover cable systems. The most current version of the DOCSIS specificationis DOCSIS 1.1, with DOCSIS 2.0 being the next planned version. In DOCSIS2.0, advanced physical layer technology is added for which some of thebenefits include more robust operation in impaired RF upstream channels.

One technical challenge in operating a network having a bidirectionalcommunication path on a shared medium between the headend and eachremote point is maintaining network integrity for signals transmitted inthe forward path and return path directions. Noise and other undesirableenergy originating at one remote point or at any point along the returnpath from that remote point can impair network communications for allremote points in the network. Similarly, where noise and undesirableenergy from one remote point is combined with noise and or other RFimpairments from other remote points in the network, networkcommunications are impaired.

RF impairments occur in many forms including, but not limited to,impulse and/or burst noise, common path distortion, and ingress such asinterference from radio communication and navigation signals. Impulsenoise or burst noise consists of high-power, short-duration energypulses. The high-power energy pulse results in a significant increase inthe noise floor while the short duration results in an elusivedisruption whose source or entry point into the network is difficult topinpoint.

Ingress is unwanted energy that enters a communication path from asource external to the communication path. Ingress often comprises radioand/or navigational communication signals propagated over the air thatenter a weak point in a wireline network, although it may also compriseimpulse and/or burst noise that is similarly propagated over the air toenter the network at a weak point. Weak points in the network oftenoccur where there is a shield discontinuity, improperly groundedelectrical device, or a faulty connector at or near a remote point. Whenradio frequency carriers from shortwave radio, citizen's band radio, orother broadcast sources enter the network at these weak points, theycause interference peaks at specific carrier frequencies in thecommunication path.

Common path distortion is the result of second and higher order mixingproducts from the downstream channel that couple to the upstream channeland occur when physical electromechanical connectors corrode and oxidizecreating point contact diodes.

The effect of these diodes in the return path is additional interferencethat is generally narrowband at fixed frequencies spaced at regular 6MHz intervals in the frequency spectrum.

Conventional mitigation techniques often adapt the signal via filtering,interleaving, coding, or spread-spectrum so that the capacity of theentire network is reduced to compensate for the interference. Inaddition, the adaptation of ingress filters may be complicated orcorrupted by the presence of burst noise during the adaptation cycle.Similarly, system adaptation for periodic burst noise interference maybe complicated or corrupted by an ingress talk spurt.

Furthermore, existing adaptations perform adaptation in a “blind”manner, in that the interference is not actually characterized. Rather,the adaptation is gradually increased in robustness until errors causedby the interference are eliminated. Because these adaptations often areperformed only at the physical layer, the ability of a system manageroperating at the link layer to characterize the status of the plant isgreatly diminished. Further, there is no capability to intelligentlychoose which adaptation mechanism to use and how often to update theadaptation parameters. This results in less efficiency. What is needed,then, is a system and method to detect and characterize interference ona communications channel so that only necessary adaptation techniquesare applied, and then in a manner that optimizes network efficiency forthe required level of robustness.

BRIEF SUMMARY OF THE INVENTION

The method and apparatus of the present invention permits the detectionand classification of impairments on a radio frequency (RF) modulatednetwork. In accordance with embodiments of the invention, a system, suchas a cable modem termination system (CMTS), detects an impairment on acommunication channel of a communication system, such as a hybrid fibercoaxial (HFC) network. The CMTS characterizes the impairment andoperating parameters of the communication system are adapted inaccordance with the characterization of the impairment. For example, theCMTS may schedule data transmissions at a frequency without theimpairment.

Alternately, the CMTS may reduce the symbol rate of the transmissionsand/or increase the error control coding in order to adapt to impulseimpairments.

In embodiments, an impairment is detected by performing a time domain tofrequency domain conversion on a signal associated with thecommunication channel. A signal magnitude is aggregated in the frequencydomain, and the signal magnitude at a set of specified frequencies iscompared with threshold values each associated with a frequency from theset of specified frequencies. A list of frequencies from the set ofspecified frequencies for which the signal magnitude of an individualfrequency exceeds a threshold value associated with the individualfrequency is reported.

In alternate embodiments, an impairment is detected by performing a timedomain to frequency domain conversion on a signal associated with thecommunication channel.

A signal magnitude is aggregated in the frequency domain, and the signalmagnitude aggregated within specific frequency ranges is compared withthreshold values each associated with a frequency range from thespecified frequency ranges. A list of frequency ranges from thespecified frequency ranges for which the signal magnitude of anindividual frequency range exceeds a threshold value associated with theindividual frequency range is reported.

In further embodiments, an impairment is detected by performing a timedomain to frequency domain conversion on a signal associated with thecommunication channel.

A pulse width and a time between pulses for the impairment signal withinone or more frequency ranges is computed, either by using successiveshort duration time domain blocks to calculate the frequency domain as afunction of time, or by selecting specific frequency ranges wheredesired and known ingress signals are not found, and reconverting thenow filtered samples back to the time domain. A periodic impairmentsignal is then detected within the one or more frequency ranges.

In still further embodiments, an impairment is detected by examiningdata packets received from the communication channel, determiningwhether the data packets contain data errors, and detecting a periodwith which data packets containing data errors arrive from thecommunication channel. This process can involve either errors in theentire packet, or errors in specific packet components such asforward-error correction (FEC) blocks.

Embodiments of the present invention examine known frequencies forcommon path distortion impairments. In an embodiment, the detection of acommon path distortion (CPD) impairment triggers a system manager tonotify the user. In response, upstream transmissions on thecommunication system can be reassigned to new frequencies and/or othersignaling parameters may be modified, such as symbol rate and FECparameters.

Embodiments of the present invention also detect periodic impairments ona path of the communication system. In embodiments, the detection ofperiodic impairments triggers the system manager to schedule no trafficduring impairment periods, schedule low priority traffic duringimpairment periods, and/or adjust the amplitude of traffic transmittedduring an impairment period.

Embodiments of the present invention also detect non-periodicimpairments on a path of the communication system. In embodiments,impulse and/or burst noise periodic and non-periodic impairments areclassified to assist an operator of the system in determining thephysical source of the impairment.

The described exemplary detection system uses a conversion process thatmaintains a power versus time characteristic of both impulse/burst noiseas well as ingress and CPD interference so that power fluctuations areobserved. An exemplary detection system calculates the rising andfalling edges from an impairment pulse waveform and then calculatespulse widths as well as the time between pulses. In addition, anexemplary detection system calculates power fluctuations within pulsesand between pulses. The described exemplary detection system furtherprocesses the pulse data to identify minimum pulse width, minimum timebetween pulse, frequency spectrum of individual pulses and maximum powerfluctuation. The average time between pulses can also be determined frompulse repetition frequency (PRF) lines in the frequency spectrum. Anexemplary detection system repeats the process for each ingress and/orCPD frequency discovered in band. The detection system preferablyreports the overall minimum and maximum pulse widths, minimum andmaximum time between pulses, and power fluctuation to the system managerfor adaptation such as modifying the center frequency, signaling rate,FEC parameters, modulation order, or other signaling parameters.

A preferred embodiment of the present invention provides detailedcharacterization of the actual RF impairments present for more efficientadaptation and for reporting plant maintenance issues to operators ofcable modem systems.

Further embodiments, features, and advantages of the present invention,as well as the structure and operation of various embodiments of thepresent invention, are described in detail below with reference to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

The accompanying drawings, which are incorporated herein and form a partof the specification, illustrate the present invention and, togetherwith the description, further serve to explain the principles of theinvention and to enable a person skilled in the pertinent art to makeand use the invention.

In the drawings:

FIG. 1 is a block diagram of a cable modem system that includes an RFimpairment detection system in accordance with embodiments of thepresent invention.

FIG. 2 is a block diagram of a cable modem termination system (CMTS) inaccordance with embodiments of the present invention.

FIG. 3 is a flow diagram demonstrating the operation of a common pathdistortion (CPD) detection system in accordance with embodiments of thepresent invention.

FIG. 4 illustrates a downstream frequency spectrum of an exemplary cableplant.

FIG. 5 illustrates a spectrum of main CPD frequencies for a harmonicallyrelated carrier (HRC) plant in accordance embodiments of the presentinvention.

FIG. 6 depicts a spectrum of main CPD frequencies for an incrementallyrelated carrier (IRC) plant in accordance with embodiments of thepresent invention.

FIG. 7 illustrates a spectrum of main and sideband CPD frequenciesresulting from the use of offset carriers in accordance with embodimentsof the present invention.

FIG. 8 illustrates a sideband frequency structure about a single mainCPD frequency in accordance with embodiments of the present invention.

FIG. 9 is a flow diagram demonstrating the operation of a CPD detectionsystem when the system does not use a frequency conversion process(e.g., a Fast Fourier Transform (FFT)) on measured data in accordancewith embodiments of the present invention.

FIG. 10 is a flow diagram demonstrating the operation of a system fordetecting periodic impulse/burst (PIB) noise in accordance withembodiments of the present invention.

FIG. 11 illustrates capture intervals within an impulse train that areexamined in a PIB detection system in accordance with embodiments of thepresent invention.

FIG. 12 is a flow diagram demonstrating the operation of a system fordetecting periodic impulse burst noise in accordance with embodiments ofthe present invention.

FIG. 13 is a flow diagram demonstrating the operation of a system fordetecting ingress in accordance with embodiments of the presentinvention.

The present invention will now be described with reference to theaccompanying drawings. In the drawings, like reference numbers indicateidentical or functionally similar elements.

DETAILED DESCRIPTION OF THE INVENTION

I. Overview

An exemplary embodiment of the present invention provides a method andapparatus for detecting and classifying RF impairments in acommunication network, such as a shared access communication network. Inan embodiment of the present invention the communication network is acable modem system. In operation, an exemplary embodiment of the presentinvention first identifies unambiguous impairments and thensystematically identifies remaining impairments.

In order to appreciate the advantages of the present invention, it willbe beneficial to describe the invention in the context of an exemplarybi-directional communication network, such as for example, a hybridfiber coaxial (HFC) network. Description in these terms is provided forconvenience only. It is not intended that the invention be limited toapplication in this example environment. Based on the teachings provideherein, persons skilled in the art will be able to implement theinvention in alternative environments.

II. Cable Modem System

A block diagram of an example cable modem system in which embodiments ofthe present invention may operate is depicted in FIG. 1. An exemplaryDOCSIS-compatible network 100 includes a headend 102 having a cablemodem termination system (CMTS) 104 located at a cable company facility.The CMTS 104 functions as a modem that services a plurality ofsubscribers. Each subscriber has at least one customer premisesequipment, such as a cable modem 106, connected to the CMTS 104 via ahybrid fiber coaxial (HFC) network 108. An exemplary CMTS for use withthe present invention is disclosed in U.S. patent application Ser. No.09/574,558, entitled “Cable Modem Apparatus and Method,” filed May 19,2000, which is incorporated by reference in its entirety.

III. Cable Modem Termination System

Referring to FIG. 2, the CMTS 104 includes a downstream modulator 204for facilitating the transmission of data communications to a pluralityof cable modems and an upstream demodulator 206 for facilitating thereception of data communications from the cable modems. On a given cableupstream upstream channel, a plurality of RF impairments maysimultaneously exist, including for example, common path distortion(CPD), periodic impulse/burst noise (PIB), and ingress. In operation, itis difficult to adapt ingress cancellation filters if burst noise ispresent during adaptation update cycles. Similarly, an ingresstalk-spurt that turns on when the system is trying to identify or adaptto PIB noise may also make correction difficult.

Therefore, in accordance with embodiments of the present invention, theexemplary CMTS 104 includes a processing core 216, such as a MIPS core,that includes an RF Impairment Detector and Classifier (IDC) 220. The RFIDC 220 comprises a software module that identifies RF impairments onthe upstream channel.

The exemplary CMTS 104 utilizes a burst receiver, rather than acontinuous receiver, to receive time division multiple access (TDMA)data packets from cable modems via upstream communication channels. Theburst receiver of the CMTS 104 comprises an analog front-end 202 havingan analog-to-digital converter (not shown) that receives analog datapackets from an upstream channel and converts the analog data packetsinto digital data packets. The upstream demodulator 206 amplifies thedigitized data packets and demodulates the amplified signal withrecovered clock and carrier timing. Matched filters and adaptive filtersremove multi-path propagation effects and narrowband co-channelinterference. An integrated decoder performs error correction andforwards the processed data in either parallel or serial MPEG-2 formatto a DOCSIS media access control (MAC) 210. The DOCSIS MAC 210 extractsDOCSIS MAC frames from MPEG-2 frames, processes MAC headers, and filtersand processes messages and data. Upstream data packets and messagepackets are then placed in system memory 214 via an internal system bus(ISB) 212.

The RF IDC 220 interfaces with a scheduler 218 and a system manager 222which comprise additional software components within the processing core216. The RF IDC 220 accepts data forwarded to system memory 214 by theupstream demodulator 206 and DOCSIS MAC 210 and, if available, FastFourier Transform (FFT)/time samples in implementations having anFFT/time sample block 208. The RF IDC 220 examines a variety of dataincluding, but not limited to, forward error correction (FEC) errors,packets that contain errors, FFT output data, time sample data, andsignal-to-noise (SNR) in order to identify when particular RFimpairments are present. Based on the RF impairments detected, thescheduler 218 and the system manager 222 are supplied with data used toadapt the system operation to increase its robustness, efficiency,capacity, or any combination of these.

Although the above description is made with reference to a softwaresystem inside a processing core, one of skill in the present art will beable to construct the above system based on the disclosure containedherein in a number of ways including, but not limited to, hardware,firmware, software, or any combination of these elements. Additionallyor alternatively, each or all of the impairments detected may bedetected and characterized by multiple processors, hardware systems, orfirmware systems, or any combination of these elements.

A. Common Path Distortion Detection System

Because common path distortion (CPD) is generally the most stableimpairment in terms of both power and frequency, and because it isidentified by specific, fixed-frequency components, a preferredembodiment of the present invention initially tests for CPD. A method300 for identifying CPD is illustrated in FIG. 3. CPD verificationbegins with a characterization of the frequency spectrum resulting fromsecond and third order mixing 302. This characterization uses thespecification of the downstream spectrum to derive analytically the CPDspectrum which would result if CPD were present. An NTSC (NationalTelevision Standards Committee) downstream signal for example, has twomain peaks, one at the video carrier and another at the audio carrier.The audio carrier signal has a smaller amplitude than the video carriersignal and has a frequency that is 4.5 MHz greater than the frequency ofthe video carrier. Thus, if f_(V) is the frequency of the video carrier,the frequency of the audio carrier will be f_(A)=f_(V)+4.5 MHz.

Subsequent carriers for other downstream cable channels will generallybe at f_(V)+m*6 MHz, f_(A)+m*6 MHz, where m=1, 2, 3, . . . , withincertain known frequency bands. The sum and difference frequencies,f_(j)−f_(i), are used to determine the CPD frequencies that result fromsecond order mixing products. Both positive and negative frequencies ofthe original spectrum are considered. The result of this computation isCPD beat frequencies at 6, 12, 18, . . . m*6 MHz, with sidebands at ±1.5MHz around every 6 MHz beat.

Thus, main, or coarse, CPD frequencies from second order mixing productsin the upstream band exist at 6.0, 7.5, 10.5, 12.0, 13.5, 16.5, 18.0,19.5, 22.5, 24.0, 25.5, 28.5, 30.0, 31.5, 34.5, 36.0, 37.5, 40.5, and42.0 MHz. Because these CPD frequencies are invariant to downstreamcarrier shifts from harmonically related carrier (HRC), incrementallyrelated carrier (IRC) or standard carrier (STD) plans, these frequencieswill always be present when CPD exists. In an embodiment of the presentinvention, the relative amplitudes of the CPD frequencies are determinedto provide greater detail in modeling and comparison to measurements bya formalism for the above computation. The formalism is based on thefact that multiplication in the time domain is equivalent to convolutionin the frequency domain. Because the frequency domain representation ofa real carrier at f_(V) is ½[δ(f+f_(V))+δ(f−f_(V))], where δ is theDirac delta function, if we represent the entire cable downstreamspectrum as only the video and audio carriers, the normalized spectrumcan be written as follows:S(f)=Σ{[δ(f+f _(i))+δ(f−f _(i))]+α[δ(f+f _(i)+4.5)+δ(f−f _(i)−4.5)]}

where the summation goes from i=1 to N_(c), N_(c) is the number ofdownstream cable channels, f_(i) is the i^(th) video carrier frequency,α is the amplitude of the audio carrier relative to the video carrier(−8.5 dB). The frequency spectrum is graphically illustrated in FIG. 4where a 6 MHz spacing between video carriers is assumed. In differentdownstream frequency plans, the spacing between video carriers istypically 6 MHz, but varies depending on the specific plan used on thecable plant. The second order mixing products are then determined bycomputing:S ₂(f)=S(f)*S(f) where * denotes convolution.

In a further embodiment, a similar approach is used to derive the thirdorder mixing products, with the result that additional frequencies at2f_(j)−f_(i) and f_(j)−2f_(i) are produced in the spectrum:S ₃(f)=S ₂(f)*S(f)=S(f)*S(f)*S(f)

For HRC systems, the additional frequencies due to third order mixingproducts are multiples of 1.5 MHz since the original carriers are atmultiples of 6 MHz+either 0 or 4.5 MHz, and twice either the videocarrier or the audio carrier minus another carrier still results infrequencies at increments of 1.5 MHz. FIG. 5 graphically illustrates amathematically simulated frequency spectrum of second and third orderCPD. The CPD frequencies at 9, 15, 21, 27, 33, and 39 MHz are solely dueto third order products, while the remaining frequencies are due to bothsecond and third order products.

Returning to FIG. 3, the simulated CPD characterization is stored on theCMTS, as shown in step 304, for correlation with received upstreamsignals. In operation, an exemplary embodiment of the present inventionperforms a Fast Fourier Transform (FFT) on the upstream band using themaximum record length, as shown in step 306. The described exemplaryembodiment preferably coordinates with the scheduler 218 to collectsamples during periods where there are no scheduled upstreamtransmissions.

The described exemplary embodiment examines the following frequency binsfor energy, as shown in step 308: 6 MHz, 12 MHz, 18 MHz, 24 MHz, 30 MHz,and 36 MHz.

In embodiments, each frequency bin may represent a single frequency or arange of frequencies. Neglecting frequencies in-band of cable modem orother known upstream signals (the frequencies are retrieved from thesystem manager 222), if the energy within more than one of the 6 MHzfrequency bins is above a predetermined threshold, the system begins theprocess of verifying the presence of CPD on the upstream channel.

This is accomplished by examining other predicted CPD frequencies andcomparing measurements at these known frequencies. Otherwise, thedetection system assumes CPD is not present on the upstream channel, asshown in step 310.

The exemplary method for identifying CPD correlates the FFT magnitude ofbins above the predetermined threshold with the simulated frequencyspectrum of the CPD, as shown in step 312, resulting from second andthird order mixing shown in FIG. 5. In an embodiment, the correlation isnot performed over the entire frequency range. For example, thecorrelation may be limited to a total shift of approximately 1.5 MHzminus the frequency bin width. In a further embodiment, the magnitude ofeach tone in the modeled frequency spectrum shown in FIG. 5 is set tounity to eliminate multiplication steps from the correlation process.

The correlation substantially reduces the mis-classification of energyin CPD frequency bins from burst noise as CPD. The correlation resultsin a large peak at zero shift and at multiples of 1.5 MHz, with muchlower values at other shift values. The described exemplary embodimentreduces the time required to perform the correlation process bycorrelating only the frequencies at 6, 12, 18, 24, 30, and 36 MHz or theaforementioned frequencies with additional tones that are ±1.5 MHz oneither side.

In an alternate embodiment, the processing time for CPD detection isfurther reduced by reducing or eliminating multiplications from thecorrelation process and instead adding the magnitudes of a plurality ofFFT bins, preferably about six, that are spaced 6 MHz apart. Afteradding the magnitudes of the plurality of FFT bins, each bin is shiftedby one in the same direction and the process is repeated until a totalrange of either 1.5 MHz or 6 MHz is covered. Depending on whichfrequencies are examined, the correlation shift process may go from zeroshift to 1.5 MHz, 6.0 MHz, or any higher frequency.

If the correlation at zero shift (and multiples of 1.5 MHz and/or 6.0MHz if used) is large relative to other shift values, preferably in therange of four to six times the other shift values, an exemplaryembodiment of the present invention determines that CPD is present onthe upstream channel 318. Otherwise, the detection system determinesthat CPD is not present 314. One of skill in the art will appreciatethat alternate representations of the CPD may be used to perform thecorrelation process to determine whether CPD is present. These alternaterepresentations of CPD may be based on different plant frequency plans(e.g., HRC, IRC, or Standard), or on a reduced complexity model of thedownstream to simplify correlation processing.

For example, IRC plans have carrier frequencies which are offset by 0.25MHz from those of HRC plans. While the offset does not affect thelocation of the second order mixing products, it does affect thelocation of third order mixing products. For example, in an IRC orStandard plant, the frequency of the audio carrier of Channel 19 is151.25+4.5=155.75 MHz. Two times the frequency of the video carrier ofChannel 4 is 2*67.25=134.5 MHz. The difference between the two is 21.25MHz. Thus, referring to the CPD frequency spectrum of an IRC plantillustrated in FIG. 6, a key indication of whether the plant is HRC orStandard/IRC is the presence of CPD frequencies at X.25 MHz or X.75 MHzlocations. Standard and IRC plans produce these coarse CPD frequencies,while HRC plans do not. This calculation can be performed in the CMTS inorder to eliminate the need for entering actual downstream frequencies,or the plant frequency plan, into the CMTS during initialization. On theother hand, an embodiment which minimizes calculations in the CMTSinvolves the user entering the type of plant frequency plan (HRC, IRC,or Standard) into the CMTS so that the CMTS may determine which model ofCPD spectrum to use in detecting the presence of CPD.

Returning to FIG. 3, an exemplary embodiment of the present inventionexamines additional frequencies based on the plant frequency plan (STD,IRC, or HRC) to further characterize the CPD frequencies that the systemmanager 222 should avoid when frequency hopping, as shown in step 320.This step is necessary since not all frequencies predicted by the modelmay in fact be strong enough to cause interference when CPD exists on agiven plant. For example, the FCC requires cable operators to offset thecarriers in certain bands by either 25 kHz or 12.5 kHz to preventinterference with aeronautical radio communications in those bands. Theconvolution of these carrier offsets result in additional CPDfrequencies.

Second order difference frequencies between an offset carrier and anon-offset carrier produce CPD frequencies at 12.5 and 25 kHz offsetsfrom the previously predicted frequencies. Third order offset productsproduce additional CPD frequencies at 37.5 kHz, 50 kHz, 62.5 kHz, etc.from the non-offset products. These third order offset products havelower amplitudes because the number of cable channels that are offset isless than the number which are not offset.

FIG. 7 graphically illustrates the CPD frequency spectrum with offsetfrequencies on an IRC plan. The coarse structure is substantiallyidentical to that in FIG. 6.

However, referring to FIG. 8, if a single 6 MHz channel, for example 18MHz, is examined, the sidebands around each coarse CPD frequency areidentified and, if desired, avoided. These offsets differ by either 12.5KHz or 25 KHz from the nominal downstream frequencies.

Tables of plant frequency plans can be used to calculate the CPDspectrum. For example in the United States, the most common frequencyplans are Standard (STD), incrementally-related coherent (IRC), andharmonically-related coherent (HRC). In other countries, other frequencyplans may exist, and will lead to different CPD spectrum models; howeverthe method for determining the CPD spectrum is identical to thatpresented here.

Further, it is possible to identify whether the CPD frequency spectrumresults from a plant amplifier imbalance or from plant oxidation. Forexample, a plant amplifier imbalance results in third order mixingproducts dominating second order mixing products. This imbalance altersthe relative strength of CPD frequencies with respect to each other. Themeasured CPD spectrum can thus be compared with simulated CPD spectracaused by plant oxidation. Thus, in an embodiment of the presentinvention, the source of the CPD spectra is determined by correlatingthe FFT results with simulated CPD spectra from causes of CPD including,but not limited to, plant oxidation and plant amplifier imbalance. Insuch an embodiment, adjustments based on the particular plant frequencyplan (STD, IRC, or HRC) are incorporated into the simulated spectra aspreviously described.

One of ordinary skill in the art will appreciate that other methods thatrely on examination of specific CPD frequencies produced by one cause ofCPD, but not another, may also be developed. The disclosed method fordistinguishing the separate causes of CPD is by way of example only andnot by way of limitation.

Returning to FIG. 3, the detection system reports CPD frequencies andthe likely cause of the CPD to the system manager 222 in the processingcore 216, as shown in step 322 The present invention provides thescheduler 218 with the ability to avoid CPD frequencies when frequencyhopping and allows the system manager 222 to notify the cable operatorof the most likely cause of CPD.

In an embodiment, the FFT processor 208 may be part of the CMTS 104, asshown in FIG. 2. In an alternate embodiment, the FFT processor 208 isoff chip from the CMTS system 104. However, if an FFT processor is notavailable, an alternate embodiment of the present invention determinesthat CPD is present on the upstream channel in accordance with themethod illustrated in FIG. 9. The described method sets the symbol rateto a low level, as shown in step 902, for example in the range of about160-320 ksymbols/sec in a DOCSIS compatible system, and scans thereceiver through upstream frequencies, as shown in step 904. Because CPDfrequencies are stationary with respect to time, typically on the orderof minutes or even hours, the CPD spectrum (i.e., the energy captured ineach upstream frequency to which the receiver is tuned) is aggregatedover a period of time, preferably on the order of seconds or minutes.The detection system preferably scans for power/energy from the highestfrequency bin to the lowest frequency bin. The detection system scansthe receiver during periods of upstream inactivity to measure the energyat each upstream frequency, including both non-CPD frequencies, as shownin step 906, and CPD frequencies, as shown in step 908. The scanningalgorithm and bin width may vary. For example, a stepped frequency withminimum symbol rate may be used. However, it will be apparent to oneskilled in the relevant art that various scanning algorithms and binwidths may be used.

The CPD detection system compares the power/energy measurements in thekey CPD frequencies at 6, 12, 18, 24, 30, and 36 MHz to the measuredpower/energy levels in other bins where CPD is not expected to occur, asshown in step 910 (as described above and further in Table 1). Forexample, in embodiments of the present invention, if the energy in theCPD frequency bins is four to six times greater than the energy in binsnot associated with typical CPD tones for the particular frequency ofthe plant, the CPD detection system declares CPD present, as shown instep 914. Otherwise, the detection system determines that CPD is notpresent, as shown in step 912.

B. Periodic Impulse/Burst Noise Detection System

FIG. 10 is a flow chart illustrating an exemplary method 1000 fordetecting the presence of periodic impulse/burst noise (PIB) inaccordance with embodiments of the present invention. The method 1000permits further impairment detection processes to avoid scanning PIBwhen detecting and classifying ingress and updating and/or adapting aningress cancellation filter. In accordance with the exemplary method1000, an FFT of the entire upstream signal is used to gather thefrequency spectrum from which PIB is detected, as shown in step 1002.The impairment detection system then identifies ‘clear bands,’ as shownin step 1004, which comprise frequency bands without ingress and withoutactual upstream communication signals. In an embodiment, the impairmentdetection system accesses an input data table of frequencies used bycurrent upstream services to identify bands without upstreamcommunications.

In further embodiments, frequency bands without ingress are identifiedby identifying contiguous frequency bins that have energy values lessthan a predetermined threshold. To minimize the impact of existingperiodic impulse/burst noise, an exemplary detection system capturestime samples using a revisit frequency that is not a multiple of 60 Hz.This prevents the possibility of capturing the same portion of a PIBwaveform in successive captures, which could preclude detection of a PIBwaveform.

In operation, an exemplary detection system that includes FFT processorcapability utilizes a first clear band that is preferably less thanabout 20 MHz and on the order of at least 1.6 MHz wide. The detectionsystem substantially reduces the sampling frequency to substantiallyincrease the sequence length of the captured time samples from thisband. In the described exemplary embodiment, the sample record length ispreferably at least 20 ms. The exemplary detection system converts thesampled stream into a DC waveform, as shown in step 1006. In anembodiment, the detection system uses envelope power detection toconvert the sampled stream. However, other techniques may be used toconvert a stream into a DC pulse waveform as will be appreciated bypersons skilled in the art. The detection system locates the rising andfalling edges of the DC pulse waveform and calculates the pulse widthsas well as the durations between pulses, as shown in step 1008. In anembodiment, the PIB detection system may choose to notify the CMTS ofthe PIB pulse width only if the detected width is well within thecapability of the system to apply forward error correction (FEC)techniques to correct errors which result from the PIB interference. Ifthe measured PIB pulse width is larger than what can be readilycorrected, or if the system is set to optimize capacity of the network,the PIB system may be instructed to further characterize and track thePIB waveforms in order to schedule around the interference.

In an embodiment of the present invention, the detection systemtransmits additional captured samples to a microprocessor via amicroprocessor interface. The additional captured samples are used totrack pulses in the waveform. The microprocessor may utilize any one ofa number of tracking techniques known in the art, including but notlimited to early/late gate tracking from radar literature. New trackingtechniques specific to CMTS-CM interactions are also possible by usingminislot registration, FEC block numbering schemes, and the like, totrack the captured pulses, as shown in step 1010.

However, if the maximum record length of the time samples is less than20 ms, time-stamps of each time sample capture are preferably correlatedwith previous captures so that pulse width, pulse periods, jitter andother metrics can be calculated. Further, the time between captures maybe specifically scheduled by the system manager 222 so that periodiccaptures out of phase with periodic impulse/burst noise are avoided.

For example, the described exemplary detection system may ‘scan’ theavailable time-slots in a periodic impulse train of period T with amaximum record length of time samples captured τ, and a minimum timebetween captures S. In operation, the detection system preferablyexamines time slots in the impulse train such that each time slotexamined corresponds to a different position within the period of theimpulse train of period T.

In an embodiment, if the initial capture is at a relative time of 0 sec,each successive capture is preferably at the ceiling((S/T)*T+nτ), wheren is the index of each successive capture, and n=1, 2, 3, . . . . Forexample, referring to FIG. 11, the described exemplary detection systempreferably examines successive time slots in the impulse train in amethodical manner. The black zones at the top of FIG. 11 are the captureintervals that are examined.

Another method that reduces the overall time to intercept of periodicimpulse trains is to examine the next available opportunity (at time S).The detection system captures this time slot if the time slotcorresponds to an unexamined portion of the impulse train period T.Alternatively, if the time slot does not correspond to an unexaminedportion of the impulse train T, the described exemplary detection systempreferably increments the time of capture by the time width of a captureand determines if this slot corresponds to an unexamined interval. In anembodiment, the system manager 222 calculates the start times of allslots within the period T (relative to a single period) and deletesstart times from the table as they are examined. The system then movesto the next unexamined time-slot indicated by the table.

In both of these algorithms, each successive capture examines adifferent region of the period of the expected impulse train. Note thattypical periodic impulse trains due to powerline phenomena have periodsthat conform to 1/m*60 sec., where m=1, 2, 3, . . . such as 16.67 ms,8.33 ms, and so on. Therefore, one approach is to choose the lowestharmonic frequency of impulse trains, or T=16.67 ms, for the period tobe scanned in the algorithms above.

Another embodiment examines the FFT of the captured trace in detail atthe lowest frequencies, or at harmonics of 60 Hz, and searches for pulserecurrence frequency (PRF) lines in the spectrum. This may be difficultif a diplexor filters out most of the spectral energy below 5 MHz; hencethis technique may be better suited to detection of periodic impulsetrains from military signals such as radar waveforms which have higherPRF's (tens or hundreds of kilohertz) and carrier frequencies which areabout 5 MHz.

Once the impulse train has been detected, the described exemplarydetection system acquires the impulse train. In embodiments, theacquisition process uses the assumed lowest pulse recurrence interval of16.67 ms for powerline phenomena, or I/PRF for higher PRF waveformsdetected via the aforementioned PRF method, and schedules captures atmultiples of this period. The detection system classifies captureresults by denoting an energy level above a threshold as “impulsedetected”. To further classify the impulse train, captures at multiplesof twice the period and three times the period that do not overlap withmultiples of the fundamental period are examined to determine if theperiodic impulse train is at a frequency of 60 Hz, 120 Hz, etc.

An embodiment of the system takes into account the fact thatoccasionally impulses in the periodic train will either not be present,or be present at substantially reduced levels, and hence will continuethe tracking process even when impulses expected to be detected are notseen. In such embodiments, the system keeps track of the number ofconsecutively missed periodic impulses and only after this numberexceeds a predetermined threshold will the system declare that theperiodic impulse train has disappeared.

Referring back to FIG. 10, the described exemplary detection systemtracks the impulse train, as shown in step 1010. Tracking of the impulsetrain is accomplished via any number of traditional methods from Radartheory, one example of which is early/late gate tracking of range asdescribed in Introduction to Radar Systems by Skolnik, Published byMcGraw-Hill Higher Education, the content of which is incorporatedherein by reference in its entirety. In alternate embodiments, methodsbased on clock recovery and tracking are used.

After detection and tracking, the described exemplary detection systempreferably reports the presence of periodic impulse/burst noise to thesystem manager 222 for adaptation and/or avoidance in the time domain,as shown in step 1012. The report includes, for example, pulse widths,pulse width jitter, pulse period, and pulse period jitter. The systemmanager 222, in conjunction with the scheduler 218, avoids theinterference, as shown in step 1014 by not scheduling any upstreamtransmissions during intervals when interference is expected.Alternately, lower priority transmissions are scheduled during expectedimpulse intervals. These transmissions are sent with increasedrobustness by, for example, being transmitted with a lower order ofmodulation and higher forward error correction (FEC)/interleaving.

An alternate embodiment for adapting to periodic impulse/burst noiseindirectly ascertains the presence of noise via examination of FECerrors as a function of time. For example, uncorrected errored FECblocks can be tracked by the receiver and kept track of for suchpurposes. Referring to FIG. 12, the system manager 222 preferablydetects and analyzes the stream to locate contiguous groups of packets(or FEC blocks) on the upstream channel that have FEC errors followed bycontiguous groups of packets (or FEC blocks) that do not have FECerrors, as shown in step 1202 The system manager 222 further analyzesthe stream to determine whether this pattern repeats at periodicintervals related to power-line frequencies 1/60 Hz, 1/120 Hz, etc., asshown in step 1204 This determination can be made based upon timestampinformation, or based on packet or FEC block sequencing/numbering whichcan be impressed on the packet or FEC block via other means or as anatural consequence of the application for which a packet is transmitted(e.g., voice over IP, or TCP connections with packet sequence numbers).As shown in step 1208, if the pattern is periodic, the system manager222 and scheduler 218 avoid communicating in time-slots having FECerrors by using a tracking approach such as early-late gate trackingdescribed above.

In an alternate embodiment, the scheduler 218 forms a null grant zoneand shifts and/or expands and contracts the null grant zone in the timedomain. When the FEC errors on incoming packets drop below apredetermined threshold, the impulse train has been detected, acquired,and tracked. The null grant zone is gradually narrowed and FECrobustness is increased for packets scheduled near the window if thesystem desires to increase throughput. As before, if the magnitude ofinterference permits, packets are scheduled during impulse intervals,albeit with greater robustness.

Furthermore, in some implementations of DOCSIS, altering the FEC,interleaving, and/or order of modulation on a burst by burst basiswithin the same Service ID (SID) is not possible. In these instances,the scheduler 218 preferably uses different SIDs or service flows, orlogical channels (in DOCSIS 2.0) to differentiate traffic scheduledduring impulse events and outside of impulse events. For example, in anembodiment, voice traffic with minimal FEC but lower required latency isscheduled outside of impulse intervals, while data traffic with higherFEC, interleaving, and lower order of modulation is scheduled elsewhere,including zones of impulse events. In an alternate embodiment, datatraffic is scheduled during non-impulse events, while voice traffic withheavy FEC, interleaving, and low order of modulation is scheduled duringimpulse events. In embodiments, the choice may depend on one or morefactors, including but not limited to the magnitude of impulseinterference, the duty cycle of interference (pulse width τ divided bythe pulse period T), the size of voice and other short packets such asTCP ACKs (acknowledges) versus data packets, and the maximum order ofmodulation permitted in clear zones.

Finally, in an embodiment, the system first detects and tracks allperiodic impulse/burst noise which is present and continues to searchfor impulse/burst noise, and whenever detected impulse/burst noise doesnot fit into current tracked waveforms, the system classifies this noiseas random and keeps track of all randomly detected events so thatoverall FEC, interleaving, packet size, and other signaling parametersunder the control of the CMTS may be altered on a global scale toaccount for random noise events which cannot be tracked.

C. Ingress Detection System

Referring to FIG. 13, the described exemplary detection systempreferably detects and tracks ingress after CPD and periodicimpulse/burst noise have been detected, as shown in step 1302.Advantageously, the detection of ingress after the detection of CPD andPIB reduces the likelihood that frequencies having CPD and/or time-slotshaving burst noise are improperly classified as having ingress. Inoperation, the described exemplary detection system performs an in-bandFFT, as shown in step 1304 on the incoming stream, preferably during aperiod when the central controller is not receiving an upstream signaland when no periodic impulse/burst noise is expected. The describedexemplary detection system analyzes the energy/power level of theincoming stream, as shown in step 1306.

Since periodic impulse/burst noise or random impulse/burst noise havewide spectral characteristics, if more than a predetermined number ofbins are above a predetermined threshold, the described exemplarydetection system classifies the bin as a random impulse/burst noiseevent, as shown in step 1310, and recaptures ingress in a different timeinterval. For additional robustness, the described exemplary detectionsystem also correlates the received signal with the predicted in-bandCPD tones to ensure that none of the detected ingress tones are due toCPD.

When an FFT capture is free of impulse/burst noise, the detection systemtabulates frequency bins where ingress is detected (energy above apredetermined threshold) as shown at step 1312. The described exemplarydetection system interfaces with the scheduler 218 and system manager222 to schedule in-band time domain captures at a reduced sampling rateand increased record length to begin characterizing the time domaincharacteristics (on/off times) of the ingress, as shown in step 1314.

The described exemplary embodiment preferably begins with the ingressthat has the highest energy level and/or the highest bandwidth. In anembodiment, the detection system utilizes contiguous frequency bins inthe FFT at similar energy levels to locate the ingress with the highestenergy level and/or the highest bandwidth. The detection system sums thetotal energy and total bandwidth in contiguous frequency bins above apredetermined threshold.

The described exemplary detection system performs envelope detection, asshown in step 1316, of the time domain captures to produce a basebandwaveform of the ingress that provides power as a function of time,on/off intervals, etc. The detection system then converts the sampledstream into DC waveforms, as shown in step 1318, using, for example,envelope detection of power, or any one of a number of techniques knownto those skilled in the art for converting an RF stream into a DC pulsedwaveform.

The described exemplary detection system uses a conversion process thatmaintains the power versus time characteristic so that powerfluctuations are observed.

An exemplary detection system calculates the rising and falling edgesfrom the DC pulse waveform and then calculates pulse widths as well asthe time between pulses. In addition, an exemplary detection systemcalculates power fluctuations within pulses and between pulses. Thedescribed exemplary system further processes the pulse data to identifyminimum and maximum pulse width, minimum and maximum time betweenpulses, and power fluctuation. An exemplary detection system repeats theprocess for each ingress frequency discovered in band. The detectionsystem preferably reports the overall minimum and maximum pulse width,minimum and maximum time between pulses and power fluctuation to thesystem manager 222, as shown in step 1320, for adaptation and ICF updaterate specification, as shown in step 1322.

IV. Conclusion

While specific embodiments of the present invention have been describedabove, it should be understood that they have been presented by way ofexample only, and not limitation. It will be understood by those skilledin the art that various changes in form and details may be made thereinwithout departing from the spirit and scope of the invention as definedin the appended claims. Thus, the breadth and scope of the presentinvention should not be limited by any of the above-described exemplaryembodiments, but should be defined only in accordance with the followingclaims and their equivalents.

1. A method for maintaining the integrity of a communication system,comprising: isolating common path distortion (CPD) in a received signalin the communication system; isolating periodic impulse/burst (PIB)noise in the received signal; identifying ingress noise in the receivedsignal after isolating the CPD and PIB noise, thereby preventingimproper classification of CPD and PIB noise as ingress noise; andadapting operating parameters of the communication system in accordancewith the identified ingress noise, the CPD and/or the PIB noise.
 2. Themethod of claim 1, wherein the identifying step further comprises:performing a fast Fourier transform on the received signal; furtherisolating random impulse/burst (RIB) noise in the received signal; andcharacterizing time domain characteristics of the ingress noise afterisolating the RIB noise in the received signal, thereby preventingimproper classification of RIB noise as ingress noise.
 3. The method ofclaim 1, wherein the isolating CPD step further comprises:characterizing the CPD in the received signal; storing the CPDcharacterization; performing a fast Fourier transform on the receivedsignal to generate a transformed signal; comparing an energy valuewithin predetermined frequency bins of the transformed signal with afirst threshold value to determine whether CPD is present; and reportingany CPD to a system manager.
 4. The method of claim 3, wherein thecharacterizing the CPD step further comprises: obtaining a measured CPDspectrum that indicates a plant amplifier imbalance; deriving asimulated CPD spectrum caused by plant oxidation; comparing the measuredCPD spectrum with the simulated CPD spectrum to characterize the sourceof the CPD.
 5. The method of claim 3, further comprising: correlatingthe transformed signal with the stored CPD characterization to generatea correlation; comparing the correlation to a second threshold value todetermine whether additional CPD is present; and reporting anyadditional CPD to the system manager.
 6. The method of claim 1, whereinthe isolating CPD step further comprises: identifying frequency bins inthe received signal where CPD is expected; measuring energy in frequencybins where CPD is expected; measuring energy in frequency bins where CPDis not expected; comparing the energy in frequency bins where CPD isexpected with the energy in frequency bins where CPD is not expected todetermine whether CPD is present.
 7. The method of claim 1, wherein theisolating PIB noise step further comprises: performing a time domain tofrequency domain conversion on the received signal; computing a pulsewidth and a time between pulses for the received signal within one ormore frequency ranges; and detecting a periodic signal within thefrequency ranges.
 8. The method of claim 1, wherein the adapting stepfurther comprises updating an ingress cancellation filter to account foridentified ingress noise.
 9. The method of claim 1, wherein the adaptingstep further comprises not scheduling data transmission during intervalswhere PIB noise is expected.
 10. The method of claim 1, wherein theadapting step further comprises scheduling lower priority transmissionsduring expected PIB noise intervals.
 11. A system for maintaining theintegrity of a communication system comprising: an analog receiver thatreceives analog data from the communication system; an analog-to-digitalconverter coupled to the analog receiver that converts the analog datainto digitized data; a fast Fourier transform (FFT) module coupled tothe analog-to-digital converter that receives the digitized data andperforms a time domain to frequency domain conversion of the digitizeddata; a processor coupled to said FFT module that examines the converteddigitized data to isolate common path distortion (CPD) and periodicimpulse/burst (PIB) noise in a received signal in a communicationchannel of the communication system and, after isolating CPD and PIBnoise, that identifies ingress noise; and a system manager coupled tothe processor that adapts operating parameters of the communicationsystem in accordance with the identified ingress noise, CPD and/or PIBnoise.
 12. The system of claim 11, wherein the FFT module furtherperforms a fast Fourier transform on the received signal; and theprocessor further isolates random impulse/burst (RIB) noise in thereceived signal and then characterizes time domain characteristics ofthe ingress noise, thereby preventing improper classification of RIBnoise as ingress noise.
 13. The system of claim 11, wherein theprocessor characterizes the CPD in the received signal and stores theCPD characterization; and wherein the FFT module performs a fast Fouriertransform on the received signal to generate a transformed signal; andwherein the processor further compares an energy value withinpredetermined frequency bins of the transformed signal with a firstthreshold value to determine whether CPD is present, and reports any CPDto a system manager
 14. The system of claim 13, wherein the processorfurther obtains a measured CPD spectrum that indicates a plant amplifierimbalance and derives a simulated CPD spectrum caused by plantoxidation; and wherein the processor further compares the measured CPDspectrum with the simulated CPD spectrum to characterize the source ofthe CPD.
 15. The system of claim 13, wherein the processor furthercorrelates the transformed signal with the stored CPD characterizationto generate a correlation; compares the correlation to a secondthreshold value to determine whether additional CPD is present; andreports any additional CPD to the system manager.
 16. The system ofclaim 11, wherein the processor further identifies frequency bins in thereceived signal where CPD is expected; measures energy in frequency binswhere CPD is expected and in frequency bins where CPD is not expected;and compares the energy in frequency bins where CPD is expected with theenergy in frequency bins where CPD is not expected to determine whetherCPD is present.
 17. The system of claim 11, wherein to further isolatePIB noise the FFT module performs a time domain to frequency domainconversion on the received signal; and the processor computes a pulsewidth and a time between pulses for the received signal within one ormore frequency ranges to detect a periodic signal within the frequencyranges.
 18. The system of claim 11, wherein the system manager updatesan ingress cancellation filter to account for identified ingress noise.19. The system of claim 11, wherein the system manager does not scheduledata transmission during intervals where PIB noise is expected.
 20. Thesystem of claim 11, wherein the system manager schedules lower prioritytransmissions during expected PIB noise intervals.